This invention relates to a substrate holder for molecular beam epitaxy apparatus.
Molecular beam epitaxy is a technique in which single crystal layers are grown by impinging molecular beams of III-V compound semiconductors such as Ga, As, etc. on the heated substrate in ultra high vacuum. Molecular beam epitaxy technique features the following in comparison with the other liquid-phase and vapour-phase epitaxial growth technique:
Low Growth rates permit to control film thickness on a very thin mono layer scale (10 .ANG.). PA1 Low substrate temperatures help to obtain a steep doping profile. PA1 In situ observation of the process of single crystal growth is effective for research on crystal growth mechanism. PA1 (A) the thermal distribution is not uniform over the substrate surface and PA1 (B) mechanical stress remains on the substrate surface.
These features are incorporated to produce various semiconductor devices, such as the photoconductive wave path, semiconductor laser, photo IC, FET, mixer diode, varactor diode, impatt diode, luminescent semiconductor, super lattice crystal, devices concerning silicon epitaxy, etc.
The molecular beam epitaxy apparatus comprises an epitaxial film growth chamber which is kept at ultra-high vacuum, such as the order of 10.sup.-10 -10.sup.-12 Torr, and substrate heating means in the said vacuum chamber. On carrying out the epitaxial film making, the back surface of the substrate is heated by the substrate heating means within the film growth chamber. Molecular beam is emitted from a material source towards the top surface of the substrate for epitaxial film making. Atoms or molecules out of the material source are caused to impinge upon the top surface of the substrate so as to merge or combine with the substrate. The substrate temperature should be kept properly for epitaxial film growth. After the completion of epitaxial film making on the substrate, the substrate is taken out from the film growth chamber and is, then subjected to the other subsequent processes for the semiconductor device fabrication.
In the molecular beam epitaxy method, it is substantially important to keep the temperature of the substrate constant during the epitaxy film making process. To keep the substrate temperature constant and to make the temperature distribution as uniform as possible over the substrate surface is essentially needed for thin film process to provide the superior characteristics of the crystal film.
A substrate holder is used in order to place the substrate at a predetermined position in the film growth chamber. In the prior art, a few types of the substrate holders have been known.
One of the types is constructed so that the substrate may be secured to a supporting member by using adhesive metal of a low melting point. Indium and gallium have mainly been used as the said adhesive metal. In this event, there are several disadvantages as follows. Since it takes a long time for the substrate to be adhered on the supporting member, contamination of the substrate is increased during the said long time substrate preparation. Air bubbles often appear in the said adhesive metal layer between the substrate and the supporting member. Appearance of air bubbles makes it difficult to attain the required ultra-high vacuum in the growth chamber. This is because outgassing occurs from the air bubble portion in the adhesive metal layer. For substrate heating, uniform thermal distribution can not readily be accomplished over the substrate surface if the thickness of the adhesive metal layer is not uniform over the substrate surface area or if it has air bubbles in it. It is essentially important to obtain uniform thermal distribution over the substrate surface. Additionally, outgass from the adhesive metal layer, especially from the air bubble portion causes serious contamination on the growing crystal film. Furthermore, the adhesive metal particle, such as indium, may be evaporated when it is heated. Since the melting point of the adhesive metal is low, such evaporated particle out of the adhesive metal may objectionably merge into the growing film. This leads to uncontrollable and inferior characteristics of the grown crystal film. After completion of the epitaxial film growth, the adhesive metal must be chemically removed from the back surface of the substrate by using some proper acid. This chemical process tends to contaminate the surface of the epitaxially grown film.
In the other type of the substrate holders, the substrate is brought in press contact with a supporting member and fastened by a set of fixing pieces together with the supporting member. The fixing pieces are of fixing metal of high thermal conductivity. When the substrate is heated by the heating means, mechanical stress will remain on the substrate surface due to a difference of expansion coefficients between the substrate and the supporting member of the substrate holder assembly. For the fixing metal, high thermal conductivity material is selected to improve substrate heating efficiency. As a result, thermal distribution does not become uniform. Nonuniform thermal distribution and mechanical stress left on the substrate result in occurrence of crystal defects in the epitaxially grown film.
Among some kinds of crystal defects, linearly localized crystal defect line, which is called "cross-hatch", is used as an evaluation factor for the grown crystal film. The inventors have found out the fact that the crystal defects called cross-hatch mainly occur on the conditions that:
If both of the conditions are simultaneously and unpleasingly fulfilled, the "cross-hatch" generation is undesiredly accelerated. In addition, the substrate is mounted directly on a substrate holder made of Mo which has high heat resistance and is secured by the use of the fixing pieces with the fixing pieces locally brought into press contact with the substrate. Therefore, the above-mentioned conditions are simultaneously fulfilled at each press contact portion. The crystalline defect seriously appears at the press contact portion in the grown film.
As another type of the substrate holders, a substrate is directly heated without a back metal plate, so as to accomplish a quick thermal response. But, the thermal absorption coefficient of the crystal substrate material, such as GaAs, is extremely low in comparison with other usual metal. Therefore, heating efficiency is not high. Additionally, in this holder, the heating means is locally and mechanically projected onto the substrate back surface and then uniform thermal distribution can not be obtained.
As one another prior art of substrate direct heating, disclosure is made about a back coating with evaporated Mo deposition film (L. P. Erickson et al, International Conference on Molecular Beam Epitaxy, Aug. 1-3, 1983, San Francisco). In this case, Mo was selected by the reason of its high heat resistance or its high melting point. But, Mo film shows high optical reflection factor. To heat the substrate, higher irradiation power of infrared light heating is needed. Higher irradiation power causes undesirably great deal of outgassing from the peripheral portion of heating means. This may lead to higher contamination probability on grown crystal film. This Mo film coating on the back substrate surface requires surplus process for the substrate preparation.